Process for whisker-free aqueous electroless tin plating

ABSTRACT

A process of preserving solderability and inhibiting tin whisker growth of exposed copper or copper alloy surfaces on a substrate comprises the steps of preparing an immersion tin plating solution substantially free of other immersion-platable metals; applying the immersion tin plating solution to the substrate to form a tin coating on the surfaces; preparing an immersion alloy plating solution containing at least two immersion-platable metals; applying the immersion alloy plating solution to the substrate by immersing the substrate in the immersion alloy plating solution to form an alloy cap layer on the tin coating. The immersion platable metals in the immersion alloy plating solution may be at least two metals selected from tin, silver, bismuth, copper, nickel, lead, zinc, indium, palladium, platinum, gold, cadmium, ruthenium and cobalt. The immersion platable metals may be added to the solution in the form of metal salts

FIELD OF THE INVENTION

This invention relates to a process for depositing a first coating ofsubstantially pure tin and a second or subsequent cap layer of one ormore alloys of at least two immersion platable metals selected from tin,silver, bismuth, copper, nickel, lead, zinc, indium, palladium,platinum, gold, cadmium, ruthenium and cobalt onto a metal surface suchas copper or copper alloys used, e.g., for electrodes or variouselectronic circuit elements. More particularly, this invention relatesto immersion plating of a first coating of substantially pure tin and atleast one cap layer of an alloy comprising at least two of the aboveimmersion platable metals on copper, copper alloys, and other metals onprinted circuit boards by chemical displacement using an electrolessimmersion, spray, flood or cascade plating application process. Stillmore particularly, this invention relates to the use of such platingsolutions in the manufacture of printed circuit boards.

BACKGROUND OF THE INVENTION

Coatings of tin, lead, bismuth, and alloys thereof have been applied tosurfaces of copper and copper-based alloys by displacement plating suchas by immersion plating techniques. Chemical displacement plating hasbeen used in the manufacture of printed circuit boards (PCB's) andparticularly multilayer printed circuit boards. Printed circuit boardsgenerally comprise a non-conducting or dielectric layer such as afiberglass/epoxy sheet which is clad with a metal conductive layer suchas copper or a copper alloy on one or both surfaces. The metal layer onthe PCB, before processing, typically is a continuous layer of copperwhich may be interrupted by a pattern of plated through-holes linkingboth surfaces of the board. During processing, selected portions of thecopper layer are removed to form a raised copper circuit image pattern,i.e., circuitry. Multilayer PCB's are typically constructed byinter-leaving imaged conductive layers such as one containing copperwith dielectric adhesive layers such as a partially cured B-stage resin,i.e., a prepreg, into a multilayer sandwich which is then bondedtogether by applying heat and pressure. Production of these types ofPCB's is described in “Printed Circuits Handbook,” Fourth Edition,Edited by C. F. Coombs, Jr., McGraw-Hill, 1996, which is incorporatedherein by reference. Since the conductive layer with a smooth coppersurface does not bond well to the prepreg, copper surface treatmentshave been developed to increase the bond strength between the layers ofthe multilayer PCB sandwich.

In addition to problems associated with adhesion of copper to laminatingmaterials, PCBs also must be treated in order to preserve thesolderability of exposed conductors. PCBs are generally fabricated in a“finished” form by a board fabricator and shipped lacking certaincircuit elements, which are later added by an assembly house. Therefore,in a finished PCB, some circuitry must be left exposed for laterattachment, usually by soldering, of further circuit elements. It iscritical that such conductors retain good solderability. Copper andcopper alloy conductors have previously been coated with tin or tinalloys to preserve solderability. However, electrodeposited pure tin issubject to “whiskering”, a phenomenon well known in the PCB industry.Tin alloys, such as tin/lead have been used to avoid the whiskeringproblem, but have the problems of expense (alloys including preciousmetals) or environmental unfavorability (lead).

Hot air solder leveling (“HASL”) has been used as a final finish for theexposed circuitry on finished PCBs. HASL produces a thick layer, e.g.,35 μin to 60 μin, which insures long-term solderability. The drawback toHASL is that the solder, particularly tin/lead solder, assumes anon-planar dome or dome-like shape on cooling. This non-planar shapemakes subsequent placement of surface mount devices problematic.

Thin deposits (1-4 microinch (μin) deposits of immersion tin have beenused with silane in multilayer bonding processes to aid bonding betweenthe metal circuitry and the laminating materials. However, this processis self-limiting as to thickness and the deposited tin rapidly alloyswith copper.

Thick, pure electrodeposited tin is known to be prone to whiskering.

Rapid deposition, high build immersion tin (“RDHBIT”) has been suggestedas an alternative to HASL as a final finish for PCBs. RDHBIT has beenused as a replacement for HASL because it is a low cost alternative toalloys such as Ni/Au, immersion silver, immersion palladium or organicsolderability preservatives.

It has been a common belief in the industry that RDHBIT isnon-whiskering. However, the present inventor has discovered that thisbelief is erroneous, and that RDHBIT deposits from every manufacturerare subject to latent tin whiskering. When a pure or substantially puretin deposit is subjected to a temperature of 130° C. for a period of 70hours in an open air oven, it develops tin whiskers. The presentinventor has discovered that even RDHBIT develops whiskers. Thispreviously unrecognized problem with immersion tin may have contributedto unexplained failures of PCBs as a result of such whiskering, and isin need of a solution.

SUMMARY OF THE INVENTION

It has now been discovered that, by applying to a substrate, e.g., to afinished PCB, a first coating of tin from an immersion tin platingsolution followed by applying a cap layer of an alloy of at least twoimmersion platable metals onto the tin coating, a high degree ofsolderability can be preserved, and the problem of tin whiskering can beavoided.

In one embodiment, the present invention relates to a process forpreserving solderability and inhibiting tin whisker growth of tin coatedcopper or copper alloy surfaces on a substrate. The process includes thesteps of:

(A) preparing an immersion tin plating solution;

(B) applying the immersion tin plating solution to the substrate to forma tin coating on the surfaces;

(C) preparing an immersion alloy plating solution containing at leasttwo immersion-platable metals;

(D) applying the immersion alloy plating solution to the substrate toform an alloy cap layer on the tin coating. In another embodiment, thesteps (B) and (D) the solution is applied by immersing the substrate inthe respective solution. In another embodiment, the immersion platablemetals are present in the form of metal salts.

In one embodiment, the substrate is a finished printed circuit board. Inanother embodiment, the immersion-platable metals are selected from tin,silver, bismuth, copper, nickel, lead, zinc, indium, palladium,platinum, gold, cadmium, ruthenium and cobalt. In another embodiment,the at least two immersion platable metals comprise tin and silver.

In one embodiment, the tin coating has a thickness in the range fromabout 20 microinches to about 300 microinches. In another embodiment,the tin coating has a thickness in the range from about 40 microinchesto about 60 microinches.

In one embodiment, the alloy cap layer has a thickness in the range fromabout 1 microinch to about 30 microinches. In another embodiment, thealloy cap layer has a thickness in the range from about 2 microinches toabout 10 microinches.

In one embodiment, the step of applying the immersion tin platingsolution includes a plating time in the range of about 5 to about 60minutes.

In one embodiment, the step of applying the immersion alloy platingsolution includes a plating time in the range of about 1 to about 10minutes.

In one embodiment, the immersion tin plating solution comprises astannous salt, an acid selected from mineral acids, carboxylic acids andhydrocarbyl-substituted sulfonic acids, a complexing agent and water. Inanother embodiment, the stannous salt is a stannous salt of ahydrocarbyl-substituted sulfonic acid and the acid is the hydrocarbyl-substituted sulfonic acid.

In one embodiment, the immersion alloy plating solution comprises atleast two immersion-platable metal salts, an acid selected from mineralacids, carboxylic acids and hydrocarbyl-substituted sulfonic acids, acomplexing agent and water. In another embodiment, each of the at leasttwo immersion platable metal salts is a salt of ahydrocarbyl-substituted sulfonic acid and the acid is thehydrocarbyl-substituted sulfonic acid.

In one embodiment, the process for preserving solderability andinhibiting tin whisker growth of tin coated copper or copper alloysurfaces on a substrate, comprising the steps of:

(A) preparing an immersion tin plating solution;

(B) applying the immersion tin plating solution to the substrate to forma tin coating on the surfaces;

(C) preparing an immersion alloy plating solution containing tin andsilver;

(D) applying the immersion alloy plating solution to the substrate toform a tin/silver alloy cap layer on the tin coating. In anotherembodiment, the tin/silver alloy cap layer comprises from about 50 wt %to about 98 wt % tin and from about 50 wt % to about 2 wt % silver. Inanother embodiment, the tin/silver alloy cap layer comprises from about80 wt % to about 95 wt % tin and from about 20 wt % to about 5 wt %silver.

In one embodiment, the printed circuit board includes electricalcircuitry formed on an outer surface of the printed circuit board, thecircuitry comprising copper or a copper alloy; a final finish on thecircuitry, the final finish comprising a coating of tin on the copper orcopper alloy circuitry; and an alloy cap layer on the tin coating, thealloy cap layer comprising at least two immersion-platable metals. Inanother embodiment, the alloy cap layer comprises an alloy of tin andsilver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The methods of the present invention preserve solderability and inhibittin whiskering of tin-coated copper or copper alloy surfaces ofsubstrates such as PCBs by use of a first immersion tin plating solutionand a second immersion alloy plating solution comprising at least twoimmersion platable metals selected from tin, silver, bismuth, copper,nickel, lead, zinc, indium, palladium, platinum, gold, cadmium,ruthenium and cobalt. From the immersion tin solution, a first coatingof immersion tin is deposited on a copper or copper alloy substrate. Theimmersion tin plating solution is substantially free of other immersionplatable metals, and, as a result, the tin coating applied to the copperor copper alloy surfaces of the PCB is substantially pure tin. From theimmersion alloy plating solution, a cap layer of an alloy of at leasttwo immersion-platable metals is deposited on the tin coating. Theimmersion alloy plating solutions contain at least two metals selectedfrom tin, silver, bismuth, copper, nickel, lead, zinc, indium,palladium, platinum, gold, cadmium, ruthenium and cobalt. The alloy caplayer comprises alloys of metals such as tin/silver, tin/lead,tin/bismuth, tin/indium, lead/bismuth, tin/germanium, indium/gallium,tin/lead/bismuth, tin/gallium/germanium, etc.

The metals are present in the respective immersion plating solutions aswater-soluble salts including the oxides, nitrates, halides, acetates,fluoborates, fluosilicates, alkane sulfonates and alkanol sulfonates ofthe respective metals. In one embodiment, the anion of the metal saltcorresponds to the anion of the acid used in the plating solution. Forexample, when fluoboric acid is used as the acid, the salts may be, forexample, stannous fluoborate, silver fluoborate, lead fluoborate,bismuth fluoborate, indium fluoborate, gallium fluoborate and germaniumfluoborate. When the acid used in the solution is ahydrocarbyl-substituted sulfonic acid, e.g., an alkane or an alkanolsulfonic acid, the soluble metal salt may be, for example, tin methanesulfonate, silver methane sulfonate, lead methane sulfonate, bismuthmethane sulfonate, indium methane sulfonate, etc., and mixtures thereofin the case of the immersion alloy plating solutions, and thecorresponding acid is methane sulfonic acid.

As used herein, the term “immersion-platable metal” is defined as ametal which can be deposited on the surface of a substrate by anelectroless plating process, for example by immersion, spraying, dippingor cascading a solution thereof on the substrate. Immersion platablemetals include at least tin, silver, bismuth, copper, nickel, lead,zinc, indium, palladium, platinum, gold, cadmium, ruthenium and cobalt.

The methods of the present invention apply generally to non-electrolyticplating methods, which may include electroless plating, immersionplating, and other forms of plating which proceed in the absence of anapplied electric potential between the plating solution and thesubstrate. Such methods include contact between the solution and thesubstrate as a result of immersion or dipping the substrate into thesolution, and spraying or cascading of the solution upon the substrate,as well as electroless plating which proceeds with the aid of asecondary reducing agent.

The method of the present invention applies a final finish to asubstrate which comprises copper or a copper alloy. In one embodiment,the final finish is applied to a PCB in which the copper or copper alloyforms the circuitry on the PCB. In another embodiment the final finishis applied to a finished PCB. As used herein, a final finish is thefinal, protective coating applied to the copper or copper alloysurfaces, i.e., the circuitry, such as conductors of a PCB, which wouldotherwise be left exposed. A finished PCB is a PCB which has beencompleted by the board fabricator, but upon which not all electrical orelectronic components have been installed or attached. Such componentsare generally installed or attached by soldering, and this installationor attachment is performed by, e.g., an assembly house which hasreceived the finished PCB from the board fabricator. The method of thepresent invention preserves the solderability of the conductors andinhibits tin whiskering of the protective tin coating during the interimbetween manufacture of the finished PCB and the installation orattachment of further components thereon by the assembly house.

The method of the present invention deposits a first, relatively heavytin coating to the substrate. The tin coating is substantially pure tin.As used herein, the term “substantially pure tin” means a tin which issubstantially free of added immersion platable metals other than tin. Asubstantially pure tin contains less than 1 wt % of other immersionplatable metals, whether such metals are added intentionally or presentas impurities in the immersion tin plating solution. More preferably, asubstantially pure tin contains less than about 0.1 wt % of otherimmersion platable metals. Thus, other immersion platable metals are notintentionally added to the immersion tin plating solution. However,small amounts of such immersion platable metals may be present asimpurities. Such amounts may be as high as 1 wt%, but should be lessthan about 0.1 wt%.

The following description of immersion plating solutions useful in themethod of the present invention is applicable both to the solutionscontaining substantially only tin (solutions of tin substantially freeof other immersion-platable metals) and to the solutions containing atleast two immersion platable metals selected from tin, silver, bismuth,copper, nickel, lead, zinc, indium, palladium, platinum, gold, cadmium,ruthenium and cobalt, except as may be otherwise specified with respectto each particular immersion plating solution.

In one embodiment, the immersion tin plating solution comprises astannous salt, an acid selected from mineral acids, carboxylic acids andhydrocarbyl-substituted sulfonic acids, a complexing agent and water. Inanother embodiment, the stannous salt is a stannoushydrocarbyl-substituted sulfonate and the acid is ahydrocarbyl-substituted sulfonic acid.

In one embodiment, the immersion alloy plating solution comprises atleast two immersion-platable metal salts, an acid selected from mineralacids, carboxylic acids and hydrocarbyl-substituted sulfonic acids, acomplexing agent and water. In another embodiment, theimmersion-platable metal salts are salts of a hydrocarbyl-substitutedsulfonate and the acid is a hydrocarbyl-substituted sulfonic acid. Inanother embodiment, the immersion-platable metal salts are silver saltsand tin salts and the alloy cap layer is an alloy of tin and silver.

The amount of tin (as tin metal) in the immersion tin plating solutionsof the present invention may be varied over a wide range such as fromabout 1 to about 120 grams of metal per liter of solution (g/l), or upto the solubility limit of the particular tin salt in the particularsolution. In one embodiment, the tin is present in the range from about5 g/l to about 80 g/l. In another embodiment, the tin is present in therange from about 10 g/l to about 50 g/l. In another embodiment, the tinis present in an amount from about 20 g/l to about 40 g/l. In anotherembodiment, the tin is present in an amount of about 30 g/l. In anotherembodiment, the tin is present in an amount of about 20 g/l. Higherlevels of tin may be included in the plating solutions, but economicssuggest, and solubility may dictate, that the metal levels be maintainedat lower levels. It should be understood that the foregoing quantitiesof tin in the immersion tin plating solution are disclosed as metallictin, but that the tin may be added to the solutions in the form of tincompounds. Such compounds may include, for example, tin oxide, tinsalts, or other soluble tin compounds. In one embodiment, the tincompounds are tin salts.

The amount of each of the at least two immersion platable metals in theimmersion alloy plating baths is in the range from about 2 g/l to about120 g/l. In one embodiment, each immersion platable metal is present inthe range from about 3 g/l to about 50 g/l. In another embodiment, eachimmersion platable metal is present in an amount from about 5 g/l toabout 20 g/l. In one embodiment, each immersion platable metal ispresent in the range from about 10 g/l to about 15 g/l. In anotherembodiment, each immersion platable metal is present in an amount fromabout 5 g/l to about 15 g/l. In another embodiment, each immersionplatable metal is present in an amount of about 10 g/l. In anotherembodiment, each immersion platable metal is present in an amount ofabout 5 g/l. Higher levels of each immersion platable metal may beincluded in the plating solutions, but economics suggest, and solubilitymay dictate, that the immersion platable metal levels be maintained atthe lower levels. It should be understood that the foregoing quantitiesof each immersion platable metal in the immersion alloy platingsolutions are disclosed as being in the metallic form, but that theimmersion platable metals may be added to the solutions in the form ofmetal compounds. Such compounds may include, for example, metal oxides,metal salts, or other soluble metallic compounds. In one embodiment, theimmersion platable metal compounds are metal salts.

The relative amounts of each of the at least two immersion-platablemetals in the alloy cap layer range from about 1 wt % to about 99 wt %each. In one embodiment, the alloy is a tin alloy and comprises fromabout 50 wt % to about 99 wt % tin. In another embodiment, the alloy isa tin alloy and comprises from about 80 wt % to about 98 wt % tin. Inone embodiment, the alloy is a tin alloy and comprises from about 90 wt% to about 95 wt % tin. In another embodiment, the alloy is an alloy oftin and silver. In another embodiment, the alloy contains from about 50wt % to about 99 wt % silver, with the remainder tin. In anotherembodiment, the alloy contains from about 90 wt % to about 97 wt %silver, with the remainder tin. In another embodiment, the alloycontains about 95 wt% silver, with the remainder tin.

A second component of the immersion plating solutions of the presentinvention is at least one acid selected from carboxylic acids, fluoboricacid, hydrocarbyl-substituted sulfonic acids,hydroxyhydrocarbyl-substituted sulfonic acids, phosphorus- andsulfur-based acids, and mixtures thereof. Such phosphorus- andsulfur-based acids include phosphoric and sulfuric acids.

The amount of acid contained in the solutions may vary from about 20 toabout 500 grams of acid per liter of solution (g/l). In one embodiment,the plating solutions contain from about 50 g/l to about 150 g/l. Inanother embodiment, one or more organic acids are present in the rangefrom about 200 g/l to about 400 g/l. In another embodiment, the organicacid is citric acid. In another embodiment, the organic acid is tartaricacid. In another embodiment, the organic acid comprises a mixture oftartaric and citric acids. In another embodiment, methane sulfonic acidis present in an amount from about 50 g/l to about 150 g/l. In anotherembodiment, methane sulfonic acid is present in an amount from about 60g/l to about 100 g/l. In another embodiment, methane sulfonic acid ispresent in an amount of about 70 g/l. In another embodiment, methanesulfonic acid is present in an amount of about 100 g/l. In anotherembodiment, fluoboric acid is present in an amount of about 70 g/l. Inanother embodiment, fluoboric acid is present in an amount of about 100g/l. In another embodiment, sulfuric acid is present in an amount ofabout 150 g/l.

Sufficient acid is present in the aqueous plating solutions to providethe solution with a pH of from about 0 to about 3. In one embodiment,the pH is from about 0 to about 2. In another embodiment, the pH is fromabout 0 to about 1. In another embodiment, the pH is in the range fromabout 0 to about -1. Generally, it is desirable to use an acid that hasan anion common to the acid salts of the metals.

The alkane sulfonic acids which are useful in the present invention asthe anion of the metal salts or as the acid component may be representedby the following Formula I

RSO₃H  (I)

wherein

R is an alkyl group containing from about 1 to about 1 2 carbon atomsand more preferably, from about 1 to 6 carbon atoms. Examples of suchalkane sulfonic acids include, for example, methane sulfonic acid,ethane sulfonic acid, propane sulfonic acid, 2-propane sulfonic acid,butane sulfonic acid, 2-butane sulfonic acid, pentane sulfonic acid,hexane sulfonic acid, decane sulfonic acid and dodecane sulfonic acid.Immersion platable metal salts of the individual alkane sulfonic acidsor mixtures of any of the above alkane sulfonic acids can be utilized inthe immersion plating solutions of the invention.

The alkanol sulfonic acids may be represented by the following FormulaII:

 C_(n)H_(2n+)—CH(OH)—(CH₂)_(m)—SO₃H  (II)

wherein

n is from 0 to about 10, m is from 1 to about 11 and the sum of m+n isfrom 1 up to about 12. As can be seen from the above Formula II, thehydroxy group may be a terminal or internal hydroxy group. Examples ofuseful alkanol sulfonic acids include 2-hydroxy ethyl-1-sulfonic acid,1-hydroxy propyl-2-sulfonic acid, 2-hydroxy propyl-1-sulfonic acid,3-hydroxy propyl-1 -sulfonic acid, 2-hydroxy butyl-1 -sulfonic acid,4-hydroxy butyl-1-sulfonic acid, 2-hydroxy-pentyl-1-sulfonic acid,4-hydroxy-pentyl-1 -sulfonic acid, 2-hydroxy-hexyl-1 -sulfonic acid,2-hydroxy decyl-1 -sulfonic acid, 2-hydroxy dodecyl-1-sulfonic acid.

The alkane sulfonic acids and alkanol sulfonic acids are availablecommercially and can also be prepared by a variety of methods known inthe art. One method comprises the catalytic oxidation of mercaptans oraliphatic sulfides having the formula R₁S_(n)R₂ wherein R₁ or R₂ arealkyl groups and n is a positive integer between 1 and 6. Air or oxygenmay be used as the oxidizing agent, and various nitrogen oxides can beemployed as catalysts. The oxidation generally is effected attemperatures below about 150° C. Such oxidation processes are describedand claimed in U.S. Pat. Nos. 2,433,395 and 2,433,396. Alternatively,chlorine can be used as the oxidizing agent. The metal salts of suchacids are prepared, for example, by dissolving a metal oxide in a hotconcentrated aqueous solution of an alkane or alkanol sulfonic acid.Mixtures of any of the above-described acids may be used, and an exampleof a useful mixture is a mixture of fluoboric acid and methane sulfonicacid.

A third component of the immersion plating solutions of the presentinvention is a complexing agent. The complexing agent may be animidazole-2-thione compound of the Formula III:

wherein

A and B are the same or different —RY groups wherein R is a linear,branched or cyclic hydrocarbylene group containing up to 12 carbonatoms, and Y is a hydrogen, halogen, cyano, vinyl phenyl, or ethermoiety.

In one embodiment, the complexing agent is a1,3-dialkylimidazole-2-thione compound (where A and B are eachindividually alkyl or cycloalkyl groups), and the thione compound may beunsymmetrical (A and B are different) or symmetrical (A and B are thesame). In another embodiment, the complexing agents are unsymmetricalsuch as (where A is methyl or ethyl and B is an alkyl or cycloalkylgroup containing from 3 to 6 carbon atoms). In another embodiment, whenA is methyl, B is a C₃-C₆ alkyl or cycloalkyl group, and when A isethyl, B is a C₄-C₆ alkyl or cycloalkyl group. An example of anunsymmetrical compound is 1-methyl-3-propylimidazole-2-thione.

Alternatively, symmetrical 1,3-dialkylimidazole-2-thione compounds maybe utilized in the immersion plating solutions of the present invention,and the dialkyl groups are the same alkyl or cycloalkyl groupscontaining from 1 to 6 carbon atoms. An example of this class ofcomplexing agents is 1,3-dimethylimidazole-2-thione.

In another embodiment, the complexing agent is thiourea, structure (IV):

In other embodiments, the complexing agent may be a thiourea compound.Thiourea compounds include those disclosed in U.S. Pat. No. 4,715,894,which is incorporated herein by reference. Suitable thiourea compoundscomprise thiourea and the various art known derivatives, homologs, oranalogs thereof.

Compounds that may be employed in this respect comprise2,4-dithiobiuret; 2,4,6-trithiotriuret; alkoxy ethers of isothiourea;thiocyanuric acid (trimer of thiourea); thioammelide (trimer ofthiourea); monoalkyl or dialkyl thiourea, where the alkyl groupcomprises a lower alkyl group, having up to about four carbon atoms suchas diethyl thiourea or monoethyl thiourea; saturated or unsaturatedcyclic hydrocarbons mono- or di-substituted thioureas such as naphthylthiourea, diphenyl thiourea, cyclohexyl thiourea and the like, where thecyclic hydrocarbon has up to about ten carbon atoms; the disulfide ofthiourea; thio-imidol (the reaction product of thiourea and sodiumhydroxide); thiocarbamic acid esters (the reaction products of thioureaand an alcohol comprising R⁴OH where R⁴ is a saturated or unsaturatedaliphatic or cyclic group having up to about ten carbon atoms) theoligomers of thiourea and formaldehyde, e.g. monomethylol, dimethylol,and trimethylol thioureas; S-alkyl pseudo thioureas (manufactured by thereaction of thiourea with an iodo- lower alkane such as iodomethanewhere the lower alkyl group contains up to about five carbon atoms);thiocarbonic acid esters of thiourea and R⁵OH, (where R⁵ is defined as(a) alkyl or alkenyl having up to about 6 carbon atoms; (b) aryl,aralkyl, or alkaryl having up to about 12 carbon atoms; or (c)cycloalkyl, cycloalkylalkyl, or alkylcycloalkyl having up to about 12carbon atoms), and especially where R⁵ is lower alkyl; thioureadioxide(also known a formamidinesulfinic acid [C.A. Reg. No. 1758-73-2]); thereaction product of a saturated or unsaturated aliphatic or cyclicorganic acid having up to about 12 carbon atoms and especially the loweraliphatic monocarboxylic acid reaction products with thiourea, e.g.,acylthioureas, and the mineral acid salts of thiourea e.g. thioureamono- or di-sulfate.

The amount of complexing agents included in the immersion platingsolutions of the present invention may range from about 5 g/l up to thesolubility limit of the complexing agent in the plating solution.Generally, the immersion plating solution will contain the complexingagent in an amount from about 5 g/l to about 150 g/l. In one embodiment,the complexing agent is present in an amount from about 50 g/l to about140 g/l. In another embodiment, the complexing agent is present in anamount from about 80 g/l to about 120 g/l. In another embodiment, thecomplexing agent is thiourea, which is present in an amount of about 100g/l. In another embodiment, the complexing agent is thiourea, which ispresent in an amount of about 125 g/l. In another embodiment, thecomplexing agent is 1-methyl-3-propyl-imidazole-2-thione, which ispresent in an amount of about 80 g/l. When the solubility of thecomplexing agent is low, a cosolvent may be added to solubilize thecomplexing agent and thereby enhance its activity in the resultingsolution. Suitable cosolvents include water-miscible solvents such asalcohols (e.g., ethanol), glycols (e.g., ethylene glycol), alkoxyalkanols (e.g., 2-butoxy ethanol), ketones (e.g., acetone), aproticsolvents (e.g., dimethylformamide, dimethylsulfoxide, acetonitrile,etc.), etc.

It is within the scope of the invention to optionally include in theimmersion plating solution, one or more surfactants compatible with eachof the solution soluble metal salts, the acids and the complexingagents. The plating solutions of the present invention may optionallycontain at least one surfactant. As noted above, the surfactant may beat least one surfactant including amphoteric, nonionic, cationic, oranionic surfactants; or mixtures thereof. More often, the surfactant isat least one cationic or nonionic surfactant; or mixtures thereof. Thenonionic surfactants are preferred.

A variety of nonionic surfactants which can be utilized in the presentinvention are the condensation products of ethylene oxide and/orpropylene oxide with compounds containing a hydroxy, mercapto or primaryor secondary amino or amido group, or other nitrogen compound containingat least one N—H group. Examples of materials containing hydroxyl groupsinclude alkyl phenols, styrenated phenols, fatty alcohols, fatty acids,polyalkylene glycols, etc. Examples of materials containing amino groupsinclude alkylamines and polyamines, fatty acid amides, etc.

Examples of nonionic surfactants useful in the plating solutions of theinvention include ether-containing surfactants having the Formula V:

R—O—[(CH₂)_(n)O]_(x)H  (V)

wherein

R is an aryl or alkyl group containing from about 6 to 20 carbon atoms,n is two or three, and x is an integer between 2 and 100. Suchsurfactants are produced generally by treating fatty alcohols or alkylor alkoxy substituted phenols or naphthols with excess ethylene oxide orpropylene oxide. The alkyl carbon chain may contain from about 14 to 24carbon atoms and may be derived from a long chain fatty alcohol such asoleyl alcohol or stearyl alcohol.

Nonionic polyoxyethylene compounds of this type are described in U.S.Pat. No. 3,855,085. Such polyoxyethylene compounds are availablecommercially under the general trade designations SURFYNOL® by AirProducts and Chemicals, Inc. of Wayne, Pa., under the designationMACOL®, PLURONIC® or TETRONIC® by BASF Wyandotte Corp. of Wyandotte,Mich., and under the designation SURFONIC® by Huntsman Corporation ofHouston, Tex. Examples of specific polyoxyethylene condensation productsuseful in the invention include MACOL® LA-23 which is the productobtained by reacting about 23 moles of ethylene oxide with 1 mole oflauryl alcohol. Another example is “SURFYNOL® 465” which is a productobtained by reacting about 10 moles of ethylene oxide with 1 mole oftetramethyldecynediol. “SURFYNOL® 485” is the product obtained byreacting 30 moles of ethylene oxide with tetramethyidecynediol.“PLURONIC® L 35” is a product obtained by reacting 22 moles of ethyleneoxide with polypropylene glycol obtained by the condensation of 16 molesof propylene oxide. SURFONIC® N-150 is an ethoxylated alkylphenol.

Alkoxylated amine, long chain fatty amine, long chain fatty acid,alkanol amines, diamines, amides, alkanol amides and polyglycol typesurfactants known in the art are also useful. One type of aminesurfactant found particularly useful in the immersion solutions is thegroup obtained by the addition of a mixture of propylene oxide andethylene oxide to diamines. More specifically, compounds formed by theaddition of propylene oxide to ethylene diamine followed by the additionof ethylene oxide are useful and are available commercially from BASFunder the general trade designation TETRONIC®.

Carbowax-type surfactants which are polyethylene glycols havingdifferent molecular weights also are useful. For example CARBOWAX®1000has a molecular weight range of from about 950 to 1050 and contains from20 to 24 ethoxy units per molecule. CARBOWAX® 4000 has a molecularweight range of from about 3000 to 3700 and contains from 68 to 85ethoxy units per molecule. Other known nonionic glycol derivatives suchas polyalkylen glycol ethers and methoxy polyethylene glycols which areavailable commercially can be utilized as surfactants in thecompositions of the invention.

Ethylene oxide condensation products with fatty acids also are usefulnonionic surfactants. Many of these are available commercially such asunder the general trade name ETHOFAT® from Armak Chemical Division ofAkzona, Inc., Chicago, Ill. Examples include condensates of oleic acid,linoleic acid, etc. Ethylene oxide condensates of fatty acid amides,e.g., oleamide, also are available from Armak.

In some of the solutions, improved results are obtained whenpolyoxyalkylated glycols, phenols and/or naphthols are included. Forexample ethylene oxide and propylene oxide condensates with aliphaticalcohols, sorbitan alkyl esters, alkyl, alkoxy and styrenated phenolsand naphthols are useful additives. About 6 to about 40 moles of theoxide may be condensed with the above identified compound. Many of thesecondensates are available commercially under such trade names as TWEEN®from ICI America, TRITON® from Rohm & Haas Co., TERGITOL® from UnionCarbide, and IGEPAL® from General Aniline and Film Corp.

The surfactants utilized in the immersion plating solutions of thepresent invention also may be amphoteric surfactants. The preferredamphoteric surfactants include betaines and sulfobetaines, and sulfatedor sulfonated adducts of the condensation products of ethylene oxideand/or propylene oxide with an alkyl amine or diamine.

Typical betaines include lauryldimethylammonium betaine and stearyldimethylammonium betaine. Sulfated and sulfonated adducts includeTRITON® QS-15 (Rohm & Haas Co.), a sulfated adduct of an ethoxylatedalkylamine, MIRANOL® HS, a sodium salt of a sulfonated lauricderivative, MIRANOL® OS, a sodium salt of a sulfonated oleic acid, etc.Cationic surfactants also are useful in the plating solutions of thepresent invention and such surfactants may be selected from the groupconsisting of higher alkyl amine salts, quaternary ammonium salts, alkylpyridinium salts and alkyl imidazolium salts.

Cationic surfactants obtained by condensation of various amounts ofethylene oxide or propylene oxide with primary fatty amines are usefuland may be represented by the following Formula VI:

wherein

R is a fatty acid alkyl group containing from about 8 to about 22 carbonatoms, R¹ is an alkylene radical containing up to about 5 carbon atoms,R² and R³ are each independently an ethylene or propylene group, a is 0or 1, and x, y and z are each independently integers from 1 to about 30,and the sum of x, y, and z is an integer of from about 2 to about 50.

More particularly, the alkoylated amines utilized in the solutions ofthe invention are represented by the Formulae VII and VIII:

wherein

R⁴ is a fatty acid alkyl group containing from 12 to 18 carbon atoms,and x, y and z are as defined in Formula V.

The above described cationic surfactants are known in the art and areavailable from a variety of commercial sources. The surfactants of thetype represented by Formula VII can be prepared by condensing variousamounts of ethylene oxide with primary fatty amines which may be asingle amine or a mixture of amines such as are obtained by thehydrolysis of tallow oils, sperm oils, coconut oils, etc. Specificexamples of fatty acid amines containing from 8 to 22 carbon atomsinclude saturated as well as unsaturated aliphatic amines such as octylamine, decyl amine, lauryl amine, stearyl amine, oleyl amine, myristylamine, palmityl amine, dodecyl amine, and octadecyl amine.

The alkoxylated amines which are useful in the plating solutions of theinvention can be prepared as mentioned above, by condensing alkyleneoxides with the above-described primary amines by techniques known tothose in the art. Many such alkoxylated amines is commercially availablefrom a variety of sources. The alkoxylated amines of the typerepresented by Formula VII are available from the Armak ChemicalDivision of Akzona, Inc., Chicago, Ill., under the general tradedesignation ETHOMEEN®. Specific examples of such products includeETHOMEEN® C/15 which is an ethylene oxide condensate of a coconut fattyamine containing about 5 moles of ethylene oxide; ETHOMEEN® C/20 andC/25 which also are ethylene oxide condensation products from coconutfatty amine containing about 10 and 15 moles of ethylene oxide,respectively; ETHOMEEN® S/15 and S/20 which are ethylene oxidecondensation products with stearyl amine containing about 5 and 10 molesof ethylene oxide per mole of amine, respectively; and ETHOMEEN® T/15and T/25 which are ethylene oxide condensation products of tallow aminecontaining about 5 and 15 moles of ethylene oxide per mole of amine,respectively. Commercially available examples of the alkoxylated aminesof the type represented by formula (5) include ETHODUOMEEN® T/13 andT/20 which are ethylene oxide condensation products of N-tallowtrimethylene diamine containing about 3 and 10 moles of ethylene oxideper mole of diamine respectively. Another type of useful cationicsurfactant is represented by the Formula IX:

where

R is an alkyl group containing from about 8 to about 12 carbon atoms, Yis a methyl or a hydroxyl group, m and n are integers, the sum of whichis from about 2 to about 20.

The amine ethoxylate surfactants of the type represented by Formula IXexhibit the characteristics of both cationic and nonionic surfactantswith the nonionic properties increasing at the higher levels ofethoxylation. That is, as the sum of x and y increases, the ethoxylatedamine behaves more like a nonionic surfactant.

The cationic surfactant also may be:

(a) a quaternary ammonium salt of the Formula X:

 wherein

X represents a halogen, a hydroxyl group, or the residue of a C₁₋₅alkanesulfonic acid; R₁ represents alkyl group R′ and R″ represent aC₁₋₄ alkyl group; and R′″ represents a C₁₋₁₀ alkyl group or a benzylgroup;

(b) pyridinium salts represented by the general Formula XI:

 wherein

X represents a halogen, a hydroxyl group, or the residue of a C₁₋₅alkanesulfonic acid; R₁ represents a C₈₋₂₀ alkyl group; and R_(a)represents hydrogen or a C₁₋₄ alkyl group;

(c) imidazolinium salts represented by the general Formula XII:

 wherein

X represents a halogen, a hydroxyl group, or the residue of a C₁₋₁₀alkanesulfonic acid; R₁ represents a C₈₋₂₀ alkyl group; R_(b) representsa hydroxy-containing C₁₋₅ alkyl group; and R′″ represents a C₁₋₁₀ alkylgroup or a benzyl group; and

(d) higher alkyl amine salts represented by the general Formula XIII:

[R₁—NH₃]⁽⁺⁾CH₃—(CH₂)_(n)—COO⁽⁻⁾  (XIII)

 wherein

R₁ represents a C₈₋₂₀ alkyl group; and n is from about 0 to about 4.

Examples of the above described cationic surfactants, in the form ofsalts, are lauryltrimethylammonium salt, cetyltrimethylammonium salt,stearyltrimethylammonium salt, lauryldimethylethylammonium salt,octadecyldimethylethylammonium salt, dimethylbenzyllaurylammonium salt,cetyldimethylbenzylammonium salt, octadecyldimethylbenzylammonium salt,trimethylbenzylammonium salt, triethylbenzylammonium salt,hexadecylpyridinium salt, laurylpyridinium salt, dodecylpicolinium salt,1-hydroxyethyl-1-benzyl-2-laurylimidazolinium salt, 1-hydroxyethyl-1-benzyl-2-oleylimidazolinium salt, stearylamine acetate, laurylamineacetate, and octadecylamine acetate.

The surfactants also may be anionic surfactants. Examples of usefulanionic surfactants include sulfated alkyl alcohols, sulfated lowerethoxylated alkyl alcohols, and their salts such as alkali metal salts.Examples of such surfactants include sodium lauryl sulfate (Duponol C orQC from DuPont), sodium mixed long chain alcohol sulfates available fromDuPont under the designation Duponol WN, sodium octyl sulfate availablefrom Alcolac, Ltd. under the designation Sipex OLS, Sodium tridecylether sulfate (Sipex EST), sodium lauryl ether sulfate (Sipon ES),magnesium lauryl sulfate (Sipon LM), the ammonium salt of lauryl sulfate(Sipon L-22), diethanolamino lauryl sulfate (Sipon LD), sodiumdodecylbenzene sulfonate (SIPONATE® DS), etc.

The surfactant(s) may be present in an amount from about 1 g/l to about100 g/l. In one embodiment, the surfactant is present in an amount fromabout 2 g/l to about 20 g/l. In one embodiment, the surfactant ispresent in an amount from about 5 g/l to about 15 g/l. In anotherembodiment, the surfactant is present in an amount of about 5 g/l. Inanother embodiment, the surfactant is present in an amount of about 10g/l. In another embodiment, the surfactant is present in an amount ofabout 9.9 g/l. In another embodiment, no surfactant is added to eitherthe immersion tin plating solution or the immersion alloy platingsolution. In another embodiment, the surfactant is added only to theimmersion tin plating solution. In another embodiment, the surfactant isadded only to the immersion alloy plating solution.

The immersion plating solutions of the present invention may alsocontain one or more chelating agents useful in keeping the immersionplatable metal and/or displaced metal in solution. The chelating agentswhich are useful in the solutions of the present invention generallycomprise the various classes of chelating agents and specific compoundsdisclosed in Kirk-Othmer, Encyclopedia of Chemical Technology, ThirdEdition, Vol. 5, pp. 339-368. This disclosure is hereby incorporated byreference. Chelating agents that are especially preferred comprisepolyamines, aminocarboxylic acids and hydroxy carboxylic acids. Someaminocarboxylic acids that may be used compriseethylenediaminetetraacetic acid, hydroxyethyletyl-enediaminetriaceticacid, nitrilotriacetic acid, N-dihydroxyethylglycine, andethylenebis(hydroxyphenylglycine). Hydroxy carboxylic acids that may beused comprise tartaric acid, citric acid, gluconic acid and5-sulfosalicylic acid. Other useful chelating agents include polyaminessuch as ethylene diamine, dimethylglyoxime, diethylenetriamine, etc. Thechelating agents may be present in an amount in the range from about 5g/l to about 50 g/l.

Various secondary reducing agents may be included in the immersionplating solutions of the present invention. The secondary reducingagents may comprise organic aldehydes whether saturated or unsaturated,aliphatic or cyclic, having up to about 10 carbon atoms, borates,hypophosphite salts and hypophosphorous acid. Lower alkyl aldehydeshaving up to about 6 carbon atoms may be employed in this respect suchas formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and thelike. Especially preferred aldehydes comprise hydroxy aliphaticaldehydes such as glyceraldehyde, erythrose, threose, arabinose and thevarious position isomers thereof, and glucose and the various positionisomers thereof. Glucose has been found to prevent oxidation of themetal salts to a higher oxidation state, e.g., tin (II) ion to tin (IV)ion, but also as a chelating agent and is especially useful for thesereasons. Other useful secondary reducing agents include sodiumhypophosphite, hypophosphorous acid, dimethylamino borane, and otherboranes. The secondary reducing agents may be present in an amount fromabout 5 g/l to about 110 g/l. In one embodiment, the reducing agent ispresent in an amount from about 20 g/l to about 60 g/l. In oneembodiment, the reducing agent is present in an amount of about 40 g/l.

The immersion plating solutions of the present invention also maycontain urea or a urea derivative, homolog or analog thereof. Examplesof useful urea derivatives are found in U.S. Pat No. 4,657,632, which isincorporated herein by reference. Specific examples include ureanitrate, urea oxalate, 1-acetylurea, 1-benzylurea, 1-butylurea,1,1-diethylurea, 1,1-diphenylurea, 1-hydroxyurea, etc. The urea compoundmay be present in an amount from in the range from about 10 to about 125g/l.

The immersion plating solutions of the present invention also maycontain one or more amidine compounds. The amidine compounds may becharacterized by the Formula XIV:

RC(═NH)NH₂  (XIV)

and the acid salts thereof wherein R is hydrogen, a hydroxyl group, analiphatic, alicyclic, aromatic or heterocyclic group, an amino group, anamidino group, and amidinoaryl group, a carboxyalkyl group, oran—NHR′group wherein R′ is an aliphatic, alicyclic, aminoalkyl,amidoalkyl or carboxyalkyl group.

The acid salts include salts such as the formates, acetates,hydrochlorides, carbonates, etc. of such amidines. The aliphatic andalkyl groups contain from one to about 10 or more carbon atoms, whereasthe alicyclic and aromatic (or aryl) groups contain from about 6 toabout 20 or more carbon atoms, and the heterocyclic groups contain fromabout 4 to about 20 carbon atoms.

Examples of such amidines are as follows. The list is exemplary only andis not to be considered as a limitation on the above formula.

Formula Name R Hydrogen H—C(═NH)NH₂ formamidine Hydroxyl HO—C(═NH)NH₂isourea Aliphatic CH₃C(═NH)NH₂ ethanamidine

4-ethylhexanamidine Alicyclic

Cyclohexanecarboxamidine

Pyrrole-2-carboxamidine Heterocyclic

2,6-diazanaphthalene-3,7- Dicarboxamide Aromatic

2-naphthamidine

Benzamidine Amino H₂N—C(—NH)NH₂ guanidine Aminoalkyl H₂NCH₂CH₂C(═ NH)NH₂1-aminopropanamidine Amidinoalkyl H₂N(HN═ )C(CH₂)₃C(═ NH)NH₂pentanediamidine Amidino H₂N(HN═ )C—C(═ NH)NH₂ ethanediamidineAmidinoaryl H₂N(HN═ )C—C₆H₄—C(═ NH)NH₂ 1,4-benzenedicarboxamidineCarboxyalkyl H₂N (HN═ )C(CH₂)₃COOH 4-carbaminidoylbutanoicacid R¹ =Aliphatic CH₃NHC(═NH)NH₂ methyl guanidine Alicyclic

Cyclohexyl guanidine Amidino H₂N(HN═)C—NH—C(═NH)NH₂ biguanide AminoalkylH₂N(CH₂)₄NH—C(═ NH)NH₂ aminobutylguanidine Carboxylalkyl H₂N(HN═)C—NH—CH₂COOH 3-guanidinoacetic acid H₂N(HN═ )C—NH—(CH₂)₂COOH3-guanidinobutanoic acid amidoakyl H₂N(HN═)C—NH—CH₂CH₂CONH₂ 3-guanidinopropionamide Heterocyclic

2-guanidinobenzimidazole

The amount of amidine compound included in the plating baths of theinvention may vary over a wide range such as from about 1 gram per literof bath up to about 100 grams per liter of bath.

In one preferred embodiment, the plating solutions of this invention aresubstantially free of thiourea and thiourea derivatives.

The various components of the immersion plating solutions useful in themethod of the present invention may be present at the concentrationsdescribed above. In one embodiment, the immersion plating solution willcontain:

about 1 g/l to about 150 g/l of each immersion-platable metal;

about 5 g/l to about 150 g/l of the complexing agent compound; and

about 20 g/l to about 500 g/l of the acid. The solution also maycontain:

about 1 g/l to about 100 g/l of a surfactant;

about 10 g/l to about 125 g/l of a urea or amidine compound;

about 5 g/l to about 40 g/l of a chelating agent; and

about 5 g/l to about 110 g/l of a secondary reducing agent.

The solution concentrations may, of course, vary depending on theparticular plating application intended.

The immersion plating solutions useful in the method of this inventioncan be prepared by adding the components to water. The components can beadded in any order.

The present invention provides a method of preserving solderability andinhibiting tin whisker growth of tin coated copper or copper alloysurfaces on a substrate. The method comprises the steps of:

(A) preparing an immersion tin plating solution;

(B) applying the immersion tin plating solution to the substrate to forma tin coating on the surfaces;

(C) preparing an immersion alloy plating solution containing at leasttwo immersion-platable metals;

(D) applying the immersion alloy plating solution to the substrate toform an alloy cap layer on the tin coating.

The plating times may vary in accordance with the desired thickness ofthe deposited metal coating or layer. In one embodiment, the substrateis immersed in the immersion tin plating solution for a period in therange from about 5 minutes to about 60 minutes. In another embodiment,the substrate is immersed in the immersion tin plating solution for aperiod in the range from about 10 minutes to about 20 minutes.

In one embodiment, the substrate is immersed in the immersion alloyplating solution for a period in the range from about 1 minute to about10 minutes. In another embodiment, the substrate is immersed in theimmersion alloy plating solution for a period in the range from about 2minutes to about 5 minutes.

The methods of the present invention, employing the above-describedaqueous electroless plating solutions may be utilized in the plating ofsubstrates such as copper, copper alloys, and other metals byelectroless plating, also referred to as chemical displacement. Theapplication steps may include immersing or dipping the substrate in theplating solution, or may include spraying, flooding or cascading thesolution onto the substrate, or may include combinations of thesemethods.

In one embodiment, metal substrates are contacted with the platingsolutions by immersing or dipping the substrates into the platingsolutions maintained at a temperature of from about 15° C. to about 80°C. In another embodiment, the temperature of the plating solution is inthe range of from about 35° C. to about 65° C. The metal substrate to beplated generally is maintained in the immersion tin plating solution forabout 5 to about 60 minutes to provide the desired tin coating qualityand thickness such as tin coating thicknesses of from about 30 to about300 microinches. The tin-coated metal substrate to be plated generallyis maintained in the immersion alloy plating solution for about 1 toabout 10 minutes to provide the desired alloy cap layer quality andthickness such as alloy cap layer thicknesses of from about 1 to about30 microinches. The plating solutions may be stirred mechanically orultrasonically to accelerate the displacement reaction.

As noted above, the metal layers deposited with the above platingsolutions include a first coating of substantially only tin and a caplayer of an immersion platable metal alloy, which alloy includes atleast two immersion platable metals selected from tin, silver, bismuth,copper, nickel, lead, zinc, indium, palladium, platinum, gold, cadmium,ruthenium and cobalt.

In one embodiment, the tin coating has a thickness in the range fromabout 20 microinches to about 300 microinches. In another embodiment,the tin coating has a thickness in the range from about 25 microinchesto about 100 microinches. In another embodiment, the tin coating has athickness in the range from about 30 microinches to about 80microinches. In another embodiment, the tin coating has a thickness inthe range from about 40 microinches to about 60 microinches. In anotherembodiment, the tin coating has a thickness of about 50 microinches. Inanother embodiment, the tin coating has a thickness of about 45microinches.

In one embodiment, the alloy cap layer has a thickness in the range fromabout 1 microinch to about 30 microinches. In another embodiment, thealloy cap layer has a thickness in the range from about 2 microinches toabout 10 microinches. In another embodiment, the alloy cap layer has athickness in the range from about 4 microinches to about 8 microinches.In another embodiment, the alloy cap layer has a thickness in the rangefrom about 4 microinches to about 15 microinches. In another embodiment,the alloy cap layer has a thickness of about 10 microinches.

The metal layers produced by the methods of the invention are dense andadhere tightly to the underlying metal substrate, and remain free of tinwhiskers even when subjected to elevated temperatures for extendedperiods. In one embodiment, after forming the alloy cap layer, thesubstrate remains free of physical change, deposits or substances (suchas oxides of copper) which may interfere with soldering even whensubjected to elevated temperatures for extended periods. In anotherembodiment, the deposited tin coating and alloy cap layer remainssolderable and free of tin whiskering when subjected to elevatedtemperatures conditions for extended periods. As a result, thesubstrate, such as a PCB, when coated with the tin coating and alloy caplayer of the present invention, remains solderable for a period of atleast one year, as estimated based on a determination by the Siemenssolderability test. The Siemens solderability test is a standard test inthe industry in which the test substrate is subjected to a temperatureof 155° C. for a period of 4 hours. At the end of the period, thesubstrate is tested for solderability, generally by dipping thesubstrate into molten solder and observing if any conductors fail to beevenly coated with the solder.

Whether a substrate coated with tin or comprising tin develops tinwhiskering may be determined by subjecting the tin-coated substrate to atemperature of about 130° C. for a period of about 70 hours. AlthoughRDHBIT was previously considered to be whisker-free, the presentinventor discovered that a RDHBIT coating does not pass this test. Thepresent inventor has discovered that embodiments of the presentinvention pass this test.

The following provides exemplary solutions which are useable in themethod of the present invention, and provides examples which illustratethe method of the present invention. Unless otherwise indicated in thefollowing examples and elsewhere in the specification and claims, allparts and percentages are by weight, temperatures are in degreescentigrade, pressure is at or near atmospheric pressure, and all rangesand ratio limits may be combined.

The methods of the present invention provide a coating on a substratesuch as a PCB which avoids whiskering and preserves solderability. WhenPCBs coated according to the methods of the present invention aresubjected to simulated storage conditions in the above-described tests,the results of the tests show that the method of the finished PCBshaving copper or copper alloy surfaces having a final finish comprisinga first coating of tin and a cap layer of an alloy of at least twoimmersion platable metals remains solderable and whisker free, whereasthe same PCBs having copper or copper alloys coated with only a coatingof tin develop whiskers and suffer from reduced solderability, afterboth surfaces are exposed to simulated extended storage conditions.

The following illustrates some immersion platable solutions useful inthe method of the present invention. The quantities of the metals in thefollowing solution are given in the concentration of the metal (or metalion) in the solution. These metals are added in the form of theindicated compounds. For example, a solution containing 33 g/l tin isformed by dissolving 111.3 g of tin methane sulfonate in water to makeone liter of tin solution. The following solutions are exemplary only,and the invention is not so limited.

g/l Solution A tin (added as stannous methane sulfonate) 33.0 methanesulfonic acid 71.2 thiourea 125.2 tartaric acid 25.1 citric acid(anhydrous) 350.3 sodium hypophosphite 39.7 MACOL ® LA-23 9.9 water537.3 Solution B-1 tin (added as stannous methane sulfonate) 33.0 lead(added as plumbous methane sulfonate) 10.0 methane sulfonic acid 71.2thiourea 125.2 tartaric acid 25.1 citric acid (anhydrous) 350.3 sodiumhypophosphite 39.7 MACOL ® LA-23 9.9 water 537.3 Solution B-2 tin (addedas stannous methane sulfonate) 33.0 indium (added as indium methanesulfonate) 5.0 methane sulfonic acid 71.2 thiourea 125.2 tartaric acid25.1 citric acid (anhydrous) 350.3 MACOL ® LA-23 9.9 water 537.3Solution B-3 tin (added as stannous sulfate) 37.0 bismuth (added asbismuth methane sulfonate) 5.0 sulfuric acid 150.0 thiourea 110.0MACOL ® LA-23 5.0 water to 1 liter Solution B-4 bismuth (added asbismuth methane sulfonate) 5.0 tin (added as stannous fluoborate) 30.0fluoboric acid 100.0 thiourea 100.0 water to 1 liter Solution B-5 tin(added as stannous methane sulfonate) 33.0 indium (added as indiummethane sulfonate) 5.0 silver (added as silver methane sulfonate) 3.0methane sulfonic acid 71.2 thiourea 125.2 tartaric acid 25.1 citric acid(anhydrous) 350.3 MACOL ® LA-23 9.9 water to 1 liter Solution B-6bismuth (added as bismuth methane sulfonate) 3.0 tin (added as stannousmethane sulfonate) 33.0 methane sulfonic acid 100.0 citric acid(anhydrous) 300.0 1-methyl-3-propyl-imidazole-2-thione 80.0 MACOL ®LA-23 9.9 water to 1 liter Solution B-7 silver (added as silver nitrate)1.0 tin (added as stannous methane sulfonate) 1.0 methane sulfonic acid20.0 histidine 1.0 3,5-dinitrosalicylic acid 1.0 water to 1 literSolution B-8 silver (added as silver methane sulfonate) 20.0 indium(added as indium methane sulfonate) 10.0 methane sulfonic acid 100.0thiourea 100.0 water to 1 liter

The following Examples illustrate use of the above-described solutions Aand B-1 to B-8, applied to a copper or copper alloy surface. Tinwhispering test results are indicated for some of the Examples.

EXAMPLE 1

A first coating of tin is applied to the exposed copper surfaces of aPCB from Solution A, plating time 10 minutes at 65° C., resulting in acoating of tin having a thickness of 50 μin, followed by an alloy caplayer applied to the tin coating from Solution B-1, plating time 2minutes at 65° C., resulting in a layer of tin/lead alloy having athickness of 10 μin. No whiskers were produced after 70 hours at 130° C.

EXAMPLE 2

A first coating of tin is applied 1 to the exposed copper surfaces of aPCB from Solution A, plating time 9 minutes at 65° C., resulting in acoating of tin having a thickness of 45 μin, followed by an alloy caplayer applied to the tin coating from Solution B-2, plating time 2minutes at 65° C., resulting in a layer of tin/indium alloy having athickness of 10 μin. No whiskers were produced after 70 hours at 130° C.

EXAMPLE 3

A first coating of tin is applied to the exposed copper surfaces of aPCB from Solution A, plating time 10 minutes at 65° C., resulting in acoating of tin having a thickness of 50 μin, followed by an alloy caplayer applied to the tin coating from Solution B-3, plating time 2minutes at 65° C., resulting in a layer of tin/bismuth alloy having athickness of 10 μin. No whiskers were produced after 70 hours at 130° C.

EXAMPLE 4

A first coating of tin is applied to the exposed copper surfaces of aPCB from solution A, plating time 9 minutes at 65° C., resulting in acoating of tin having a thickness of 45 μin, followed by an alloy caplayer applied to the tin coating from Solution B-4, plating time 4minutes at 40° C., resulting in a layer of tin/bismuth alloy having athickness of 10 μin.

EXAMPLE 5

A first coating of tin is applied to the exposed copper surfaces of aPCB from Solution A, plating time 10 minutes at 65° C., resulting in acoating of tin having a thickness of 50 μin, followed by an alloy caplayer applied to the tin coating from Solution B-5, plating time 3minutes at 50° C., resulting in a layer of tin/indium/silver alloyhaving a thickness of 10 μin.

EXAMPLE 6

A first coating of tin is applied to the exposed copper surfaces of aPCB from Solution A, plating time 10 minutes at 65° C., resulting in acoating of tin having a thickness of 50 μin, followed by an alloy caplayer applied to the tin coating from Solution B-6, plating time 2minutes at 65° C., resulting in a layer of tin/bismuth alloy having athickness of 10 μin.

EXAMPLE 7

A first coating of tin is applied to the exposed copper surfaces of aPCB from Solution A, plating time 10 minutes at 65° C., resulting in acoating of tin having a thickness of 50 μin, followed by an alloy caplayer applied to the tin coating from Solution B-7, plating time 2minutes at 65° C., resulting in a layer of tin/silver alloy having athickness of 8 μin. The tin/silver alloy contains approximately 90 wt %silver. No whiskers were produced after 70 hours at 130° C.

EXAMPLE 8

A first coating of tin is applied to the exposed copper surfaces of aPCB from Solution A, plating time 10 minutes at 65° C., resulting in acoating of tin having a thickness of 50 μin, followed by an alloy caplayer applied to the tin coating from Solution B-8, plating time 2minutes at 65° C., resulting in a layer of tin/silver alloy having athickness of 8 μin. The tin/silver alloy contains approximately 90 wt %silver.

COMPARATIVE EXAMPLE

A single coating of tin from Solution A is applied to the exposed coppersurfaces of a PCB, plating time 10 minutes at 65° C., tin thickness 50μin.

No alloy cap layer is applied.

Whiskers were produced in less than 70 hours at 130° C.

The coating or layer deposited from the plating solutions by the methodsof the invention are useful in the electronic circuits, electronicdevices and electrical connectors. The first coating is of tin, and thesecond or subsequent cap layer of an alloy of at least two immersionplatable metals selected from tin, silver, bismuth, copper, nickel,lead, zinc, indium, palladium, platinum, gold, cadmium, ruthenium andcobalt. The coating or layer can be used as protective layers tomaintain and preserve solderability while at the same time preventingtin whiskering by the tin layer. In addition, the layers preventcorrosion of copper in a patterning procedure during the fabrication ofprinted circuits or integrated circuits. The coating or layer maintainand preserve solderability by providing chemically stable surfaces onprinted wireboards, etc.

The methods of the present invention are useful in the manufacture ofmultilayer printed circuit boards which have alternating layers ofdielectric material which support copper circuitry (which may have otherlayers such as a copper sheet interspaced which serves as a groundplane) which are adhered to an insulating layer through intermediatelayers. The circuit board has conductive through holes which formelectrical paths across the entire thickness of the board. Multilayercircuit boards may comprise several dozen conductive and non-conductivelayers. In the formation of multilayer circuit boards, it is oftennecessary to drill holes through the boards, and defects can occur dueto delamination of layers in the areas immediately surrounding a hole.

A starting material is a dielectric layer which contains a cladding ofcopper on one or both surfaces. The copper layer may have a totalthickness in the range from about 4 microns up to about 128 microns. Inone embodiment, the copper is ¼ ounce foil, i.e., it weighs ¼ ounce persquare foot, and is about 8 microns thick. In another embodiment, thecopper is ½ ounce foil, and is about 16 microns thick. In anotherembodiment, the copper is {fraction (3/4)} ounce foil, and is about 24microns thick. In another embodiment, the copper is 1 ounce foil, and isabout 32 microns thick. In another embodiment, the copper is 2 ouncefoil, and is about 64 microns thick. In another embodiment, the copperfoil has electrodeposited thereon a layer of copper about 1 micronthick. This electrodeposited copper may be used to form theinterconnections between conductive layers of the PCB through theconductive holes. The copper or copper alloy layer may be used to formconductive circuitry. Well known techniques can be employed to form suchcircuitry.

The composition of the dielectric layer is not critical provided itfunctions as an electrical insulator. After formation of the conductivecircuitry, when a finished PCB has been prepared, a final finish isapplied according to the method of the present invention. In the methodof the present invention, a first, relatively heavy, coating of tin isapplied to the exposed copper or copper alloy circuitry of the finishedPCB, followed by a second, relatively light, outer or cap layer of analloy of at least two immersion platable metals selected from tin,silver, bismuth, copper, nickel, lead, zinc, indium, palladium,platinum, gold, cadmium, ruthenium and cobalt, which is applied to thetin coating. The circuitry of the printed circuit board typically isfirst cleaned and etched before it is contacted with the platingsolutions of the present invention.

While the invention has been explained in relation to its preferredembodiments, it is to be understood that various modifications thereofwill become apparent to those skilled in the art upon reading thespecification. Therefore, it is to be understood that the inventiondisclosed herein is intended to cover such modifications as fall withinthe scope of the appended claims.

What is claimed is:
 1. A process for preserving solderability andinhibiting tin whisker growth of tin coated copper or copper alloysurfaces on a substrate, comprising the steps of: (A) preparing animmersion tin plating solution; (B) applying the immersion tin platingsolution to the substrate to form a tin coating on the surfaces; (C)preparing an immersion alloy plating solution containing at least twoimmersion-platable metals; (D) applying the immersion alloy platingsolution to the substrate to form an alloy cap layer on the tin coating.2. The process of claim 1, wherein the substrate is a finished printedcircuit board.
 3. The process of claim 1, wherein the immersion-platablemetals are selected from tin, silver, bismuth, copper, nickel, lead,zinc, indium, palladium, platinum, gold, cadmium, ruthenium and cobalt.4. The process of claim 1, wherein the at least two immersion platablemetals comprise tin and silver.
 5. The process of claim 1, wherein thetin coating has a thickness in the range from about 20 microinches toabout 300 microinches.
 6. The process of claim 1, wherein the tincoating has a thickness in the range from about 40 microinches to about60 microinches.
 7. The process of claim 1, wherein the alloy cap layerhas a thickness in the range from about 1 microinch to about 30microinches.
 8. The process of claim 1, wherein the alloy cap layer hasa thickness in the range from about 2 microinches to about 10microinches.
 9. The process of claim 1, wherein in the steps (B) and (D)the solution is applied by immersing the substrate in the respectivesolution.
 10. The process of claim 1, wherein the step of applying theimmersion tin plating solution includes a plating time in the range ofabout 5 to about 60 minutes.
 11. The process of claim 1, wherein thestep of applying the immersion alloy plating solution includes a platingtime in the range of about 1 to about 10 minutes.
 12. The process ofclaim 1, wherein the immersion tin plating solution comprises a stannoussalt, an acid selected from mineral acids, carboxylic acids andhydrocarbyl-substituted sulfonic acids, a complexing agent and water.13. The process of claim 12, wherein the stannous salt is a stannoussalt of a hydrocarbyl-substituted sulfonic acid and the acid is thehydrocarbyl-substituted sulfonic acid.
 14. The process of claim 1,wherein the immersion alloy plating solution comprises at least twoimmersion-platable metal salts, an acid selected from mineral acids,carboxylic acids and hydrocarbyl-substituted sulfonic acids, acompleting agent and water.
 15. The process of claim 14, wherein each ofthe at least two immersion platable metal salts is a salt of ahydrocarbyl-substituted sulfonic acid and the acid is thehydrocarbyl-substituted sulfonic acid.
 16. The process of claim 15,wherein the immersion-platable metal salt comprises a metal selectedfrom tin, silver, bismuth, copper, nickel, lead, zinc, indium,palladium, platinum, gold, cadmium, ruthenium and cobalt.
 17. Theprocess of claim 1, wherein the immersion platable metals are present inthe form of metal salts.
 18. A process for preserving solderability andinhibiting tin whisker growth of tin coated copper or copper alloysurfaces on a substrate, comprising the steps of: (A) preparing animmersion tin plating solution; (B) applying the immersion tin platingsolution to the substrate to form a tin coating on the surfaces; (C)preparing an immersion alloy plating solution containing tin and atleast one immersion platable metal other than tin; (D) applying theimmersion alloy plating solution to the substrate to form a tin alloycap layer on the tin coating.
 19. The process of claim 18, wherein thetin alloy cap layer comprises from about 50 wt % to about 98 wt % tin.20. The process of claim 18, wherein the tin alloy cap layer comprisesfrom about 80 wt % to about 95 wt % tin.
 21. The process of claim 18,wherein the at least one immersion platable metal is silver.
 22. Theprocess of claim 21, wherein the tin alloy cap layer comprises fromabout 50 wt % to about 98 wt % silver.
 23. The process of claim 21,wherein the tin alloy cap layer comprises from about 80 wt % to about 95wt % silver.
 24. The process of claim 18, wherein the immersion platablemetals are present in the form of metal salts.
 25. The process of claim1, wherein the immersion-platable metals are selected from tin, silver,bismuth, copper, nickel, zinc, indium, palladium, platinum, gold,cadmium, ruthenium and cobalt.
 26. The process of claim 18, wherein theimmersion-platable metal other than tin is selected from silver,bismuth, copper, nickel, zinc, indium, palladium, platinum, gold,cadmium, ruthenium and cobalt.